Avian radars are used to track birds in flight in the vicinity of airfields, wind farms, communications towers, and along migration routes. Birds are a significant hazard to aviation safety. Applications that require bird monitoring are the bird aircraft strike hazard (BASH) problem and the natural resource management (NRM) problem. Billions of dollars in damage to aircraft and significant loss of life have been recorded due to birds flying into aircraft, particularly during take-off and landing in the vicinity of airports.
The danger associated with birds depends on their altitude (among other factors). Users of bird detection and tracking radars need to know the height of tracked birds. State-of-the-art avian radars provide target tracking with localization in only two dimensions. These systems do not estimate height (within the beam extent) in any real sense. Thus avian radars need altitude estimation of bird (or other airborne) targets. They need the means to estimate target height in a manner that is practical and economical. The purpose of the current invention is to provide next generation avian radars with such means, thereby overcoming current limitations in the state-of-the-art.
State-of-the-art avian radars use inexpensive, commercial-off-the-shelf (COTS) X-band marine radar transceivers, fitted with slotted-waveguide array antennas, as well as parabolic reflector or Cassegrain (dish) antennas. The raw received baseband signals are digitized, followed by detection and tracking of bird targets. State-of-the-art avian radars provide continuous, day or night, all-weather, situational awareness with automated detection, localization and warnings of hazards. They provide high-quality target track data with sophisticated criteria to determine potentially dangerous target behavior, as well as communication of alerts to users who as require that information. They also minimize operator interaction.
State-of-the-art avian radars features include:                Low-cost, high-performance radar antennas and transceivers mounted on ground-based pedestals        Radar processing that can reliably detect and track small, low-RCS (radar cross-section), maneuvering targets in dense target and clutter environments        Automatic hazard detection and alert capability to remote users        The formation of radar networks to provide wide-area coverage        Low cost of operation        Low life cycle costs        Data and analysis support for research and development        
COTS marine radars are very inexpensive. These marine radars exhibit surprisingly good hardware specifications. However, as-is, these radars deliver mediocre performance for bird targets because of their primitive signal processing. Combining a COTS marine radar with a digitizer board and a software radar processor that runs on a COTS personal computer (PC) and a parabolic dish antenna forms a state-of-the-art avian radar, one with a very limited three-dimensional (3D) localization capability. Modifying such radars via custom antennas and processing allows height estimation and coverage.
Slotted-waveguide array antennas are used to provide two-dimensional (2D) localization (i.e. range and azimuth, which can be translated to latitude and longitude). These systems provide good volume coverage due to the typically larger vertical (elevation) beamwidth, which is on the order of 20 degrees. Such systems cannot provide useful height estimates of tracked targets when the radar is spinning horizontally in its usual orientation. This is because the beam uncertainty in the 3rd dimension (elevation), which is on the order of the beam extent, is too large. For example, the elevation beam extent or height uncertainty for a target at a distance of just 1 km from the radar is about 1,000 feet. This means that if both a plane and a bird are being tracked by the radar at a distance of 1 km away, the radar cannot tell whether the two targets are 1,000 feet apart (i.e. one is on the ground and the other is at the upper edge of the vertical beam, 1,000 feet off the ground) or whether they are at the same altitude where a collision could occur. While some radar configurations orient the slotted-array antenna so that it spins vertically (rather than horizontally) to get a measure of height, see Nocturnal Bird Migration over an Appalachian Ridge at a Proposed Wind Power Project, Mabee et al, Wildlife Society Bulletin 34(3), 2006, page 683, they still can only operate as 2D radars. In order to measure height, they can no longer provide 360-degree azimuthal coverage (which a conventional azimuth-rotating radar provides).
Parabolic reflector or Cassegrain (dish) antennas are used today to provide a very limited 3D localization capability. These antennas employ a single beam (pencil shaped), fixed in elevation, but rotating in azimuth. The azimuth rotation results in the usual 2D, 360-degree coverage with localization in range-azimuth or latitude-longitude. However, by using a narrow pencil beam (say between 2 and 4 degrees wide), the height uncertainty reduces significantly as compared to the 20 deg slotted-array antenna. Using the previous example, with targets at a distance of 1 km from the radar and a 4-degree dish antenna, height estimates with uncertainties on the order of 200 feet are now possible. While providing useful height information at very short ranges, the height estimates are still of limited use at further ranges. Also, volume coverage is restricted accordingly with the narrower pencil beam. The present invention seeks to overcome these limitations by providing better 3D localization capabilities. In particular, means are disclosed herein to provide both better height estimates (reduced height uncertainty) and greater volume coverage.
Merrill I. Skolnik in his Introduction to Radar Systems, 2nd Edition, McGraw-Hill Book Company 1980 and his Radar Handbook, 2nd Edition, McGraw-Hill, Inc., 1990, describes height-finding radars that use nodding horizontal fan beams. These radars are steered to the bearing where targets have been detected by an independent 2D air-surveillance radar. These height-finding radars can not get height estimates for more than 20 or so targets per minute, and have problems with azimuth-elevation (Az-El) ambiguities in dense target environments. Military airborne and land-based tracking radars provide height information for a single target only (via closed-loop steering in both dimensions). They use monopulse or sequential lobing techniques to obtain the off-boresight error signals, but like the height-finding radars, are unable to perform 3D surveillance. Military 3D surveillance radars, on the other hand, employ rotating phased array antennas that form either multiple receive beams or rapidly electronic-scanning pencil beams. See Radar Applications, Merrill I. Skolnik, IEEE Press New York, 1987. Like these radar systems, the present invention is also true 3D surveillance; its antenna rotates in azimuth while estimating height. However, the present invention is low-cost, while military 3D radar systems are orders of magnitude more expensive, because of their phased array antennas. The present invention does not use expensive phased arrays but uses marine radars and PC-based processing to achieve considerable cost reduction, especially as compared to military systems.
The U.S. and Canada have conceived and are developing a North-American Bird Strike Advisory System (NABSAS). This system will monitor and provide information to users on bird activity and hazards (to aircraft) at numerous sites throughout North America. It includes a network of avian radars as part of its data sources, and bird heights as well as bird ground tracks are desired. 3D avian radars in accordance with the present invention will provide ideal sources of bird information for this Advisory System.
It will be obvious to those skilled in the art that the same improvements described herein are applicable to low-cost radars used in other applications such as homeland security. Any radar with plot extraction (i.e. detection) could use the apparatus and method described herein to estimate height of detected targets. Examples of such radars are described in US Patent Application Publication No. 2006/0238406 entitled “Low-cost, High-performance Radar Networks,” which is incorporated herein by reference.